The invention is based on a method for recording ion-cyclotron-resonance spectra in which gaseous ions of a sample substance are produced and, in an ultrahigh vacuum, are exposed to a homogenous constant magnetic field and to an oscillating electric field of a given frequency, which is perpendicular to the magnetic field, a process during which ions brought to resonance by the alternating field produce a test signal and, moreover, the gaseous ions (secondary ions) of the sample substance are produced by bombardment of the sample substance with other gaseous, high-energy ions (primary ions).
The process of ion-cyclotron resonance (ICR) is known, from the DE-OS No. 31 24 465 (U.S. Pat. No. 4,464,570, for example.
In ICR-spectroscopy, which can be considered a special case of mass spectroscopy, producing gaseous ions from the molecules of a sample substance to be examined is a common problem . These gaseous ions are required because they are brought to resonance in ICR-spectroscopy.
In producing these gaseous ions, a high efficiency factor is desired on the one hand and a production process that is as gentle as possible on the other. High efficiency leads to a high ion yield and with it exposure to a test signal of relatively strong magnitude or a high degree of sensitivity for the apparatus used. The demand for as gentle a process as possible means that the molecules should reach a charged state as intact as possible and as chemically unchanged as possible. With respect to these two requirements, particular difficulties arise if the sample substance is almost nonvolatile and solid.
From the classical mass spectroscopy, in which, as is known, the ions are accelerated lineraly and not, as in ICR-spectroscopy, on a circular path, several methods are known to meet the requirements mentioned for the production of gaseous ions.
In chemical ionization (CI), very reactive primary ions are chemically brought into interaction with the already gaseous molecules of the sample substance that are to be examined. Although this process is very gentle, it has the disadvantage that the sample substance to be examined must first be evaporated, which dramatically limits the range of applications.
In direct chemical ionization (DCI), as with chemical ionization, primary ions are also used. In this case, primary ions directly act on a solid sample substance. But the disadvantage of this well-known method is its very low yield.
In laser desorption, a solid sample substance is directly exposed to a laser beam at high-energy density. In many fields of application, this method satisfies the demand for a good yield of intact molecular ions. The disadvantage of this process, however, is that it also requires a considerable amount of equipment investment.
With the field desorption method (FD), molecules of the sample substance are exposed to extremely high electric field intensities on specially manufactured electrodes so that part of these molecules are emitted as ions. This method has the disadvantage, however, that it is extremely complicated to carry it out, so that it can only be used by persons with considerable experimental experience and skill. Moreover, it is necessary to produce special electrodes first and to prepare the sample in an appropriate manner.
Finally, the fast atom bombardment method (FAB) method is known, in which the sample is bombarded with linearly accelerated, high-energy inert gas atoms (or ions). In this process, the sample substance is generally first dissolved in a glycerin matrix so that the surfaces are constantly regenerated by diffusion in a vacuum, which means that molecules of the sample substance are always present on the surface. However, the sample substances to be examined can also be applied to metal surfaces in a suitable manner, as described by A. Benninghofen and W. Sichtermann in the U.S. Journal of Applied Physics 11, pages 35-39, 1976. For this purpose, the energy of the molecules that strike the sample substance is usually in the 5,000 to 10,000 eV range.
In principle, the aforementioned methods known in mass spectroscopy can also be applied in ICR-spectroscopy if ions of almost nonvolatile, complex sample substances must be produced. However, the disadvantages described in detail with the methods are of even greater consequence in ICR-spectroscopy, because all additional equipment has to operate under ultrahigh-vacuum conditions, i.e., under vacuum conditions equivalent to a pressure approximately two orders of magnitude lower than in classical mass spectroscopy.
The invention, therefore, starts out from the task to further develop a method of the type described at the beginning to the point where, with a low investment in equipment, gaseous ions can be produced under the ultrahigh-vacuum conditions of ICR-spectroscopy, even from almost nonvolatile and complex sample substances.
In accordance with the invention, this task is accomplished by exciting the primary ions by ion-cyclotron resonance also.
The method in accordance with the invention thus has the fundamental advantage that excitation of the primary ions in accomplished in the same way as the excitation of the secondary ions that are to be measured, so that to that extent, comparable test conditions exist.
In this connection, an embodiment is preferred in which the primary ions are produced in the immediate proximity of the sample substance and primary and the secondary ions are preferably excited in the same measuring cell (analyzer), using the same resonance equipment. In this way, a considerable reduction of the equipment investment results because additional resources are only necessary insofar as the production of the primary ions is required, while the excitation of the primary ions to the required energy level can be achieved with the same equipment resources that are available anyway for the subsequent actual ICR-measurement proper.
In a preferred embodiment of the method in accordance with the invention, the primary ions are produced from an inert gas, preferably argon. In accordance with the invention, it is, however, also possible to use a chemically reactive gas instead of an inert gas.
A particularly simple set-up is accomplished if, in a more elaborate version of the invention, the primary ions are produced in a measuring cell (analyzer) by an electron beam introduced at a distance from the sample substance and the amplitude of the alternating field for the ICR of the primary ions is adjusted in such a way that for the primary ions a circular path results that runs through the location of the sample substance. In this case, all of the additional investment consists in providing means for the production of an electron beam, whose position in the space must only be adjusted with reference to the position of the sample substance in a way which suits the measuring conditions and the ions used in a given situation.
To eliminate undesirable interactions between the excited primary ions and the secondary ions that are to be measured, in further elaboration of the invention, an alternating field for the primary ions is introduced before excitation of the ICR; its amplitude must be set to a level at which the primary ions reach a collector electrode, which is preferably a mass electrode. In this way, it is possible to "clean" the measuring cell before recording the actual ICR-spectrum by introducing a simple high frequency pulse so that the primary ions are selectively removed. This selective removal of the primary ions is necessary because by applying an ordinary quench pulse, during which the ion trap is temporarily opened, the secondary ions that have just been produced would also be removed.
To carry out the procedure in accordance with the invention, it is preferred that an apparatus be used in which a sample carrier for the sample substance is situated in an ultrahigh vacuum measuring cell, the measuring cell furthermore contains an ionizable medium, the means for ionizing the medium, further means for exciting an ICR of the ions of the medium, and means for exciting and measuring the ICR of the ions of the sample substance are provided.
Along with the above, preferrably outside the measuring cell, an arrangement for producing an electron beam is provided which is introduced through openings into the measuring cell in such a way that its path runs at a preselected distance from the sample carrier.
A particularly good effect is achieved if the measuring cell has a circular cylindrical shape, with four cylinder barrel segment-shaped surfaces provided which, being situated opposited each other in pairs, form a transmitter or receiver and are DC-coupled to mass, and further two cover surfaces are connected to a finite potential to serve as ion traps. This arrangement has the advantage that because of the circular cylindrical shape of the measuring cell, efficient use is made of the space in a solenoid coil as it is used with super conductive magnets for producing high constant magnetic field intensities. Feeding the electron beam or introducing the sample in a direction parallel to the axis of the measuring cell corresponds here with a direction of particularly good access to the portion of the field of the solenoid coil that can be used for measuring purposes. The direct current coupling of the above mentioned segment surfaces has the advantage that with the above-described strong excitation of the primary ions, immediately prior to the beginning of the actual ICR experiment, the radius of their orbit becomes so great that the primary ions come in contact with the transmitter or receiver surfaces and are removed because of the grounded DC-coupling to mass. Connecting the cover surfaces to a finite potential makes it possible, depending on the polarity of this potential, to capture positive or negative ions in the measuring cell.
In further elaboration of the apparatus in accordance with the invention, the sample carrier is attached to a push rod which runs in parallel to the axis of the measuring cell, inside the latter, and can be moved in a radial direction. This arrangement has the advantage that the position of the sample carrier can be adapted in a radial direction to the appropriate measuring conditions. Because the sample substance must be located on the rotational path of the primary ions, it is easy to adapt the arrangement to the primary ions used in a particular case or the strength of the magnetic field, etc.
Additional advantages are evident in the description and the enclosed drawing.